A production process of semiconductor integrated circuits includes a film formation processing of forming insulation film such as silicon dioxide film and silicon nitrided film on semiconductor wafers. In the film formation processing, a plasma CVD technique (PECVD) is widely used. In the film formation processing, sediments, which are primarily composed of film forming constituents, are accumulated in a treatment container of a semiconductor production equipment, and if the accumulation of the sediments is not removed, the accumulated sediments fall off the surface of the treatment container and accumulate on the semiconductor wafers, which may lead to incomplete film formation and defect of a device. Therefore, the treatment container needs to be cleaned according to the frequency of the film formation processing.
As one of methods of cleaning the treatment container, a dry-cleaning method is widely used in the prior art. In the dry-cleaning method, the sediments are changed into a silicon tetrafluoride (SiF4) gas by radical fluorine atoms and discharged from the treatment container. In order to produce the radical fluorine atoms, plasma generating methods are conventionally used. In the plasma generating methods, gases containing fluoride atoms are used, and typically hexafluoroethane (C2F6) or nitrogen trifluoride (NF3) is ionized.
As such ionization methods, an in-situ plasma method and a remote plasma method are known in the prior art. In the in-situ plasma method, the treatment container of the semiconductor production equipment doubles as a plasma generating chamber. In the remote plasma method, the plasma generating chamber is arranged outside of the treatment container of the semiconductor production equipment. As compared to the in-situ plasma method, the remote plasma method has the advantages that the treatment container of the semiconductor production equipment is insulated from damages, and the degradation efficiency by supply gas is high, and cleaning time is cut down and so on. Therefore, the remote plasma method is, especially suitable for a semiconductor production equipment designed for film formation in low temperature (See also, for example Patent Document 1).
As one of plasma generating apparatuses employed for the remote plasma method, there is a microwave plasma generating apparatus which generates plasma utilizing microwave discharge. The microwave plasma generating apparatus comprises, for example, a wave guide extending into a microwave cavity and a gas transport tube extending through the microwave cavity and across an axis of the wave guide, in which a microwave generated by a high-frequency power source is supplied to the wave guide and a gas is supplied to the gas transport tube so that the gas is ionized by electric discharge generated within the gas transport tube (See also, for example, Patent Document 2).
In this microwave plasma generating apparatus, the initial setting of the apparatus is carried out in such a way that the impedance matching is caused to maximize power of the microwave at a position of the inside of the gas transport tube within the microwave cavity, and thereby a standing wave (resonant state) is maintained within the wave guide. The initial setting is also carried out in such a manner that a resonant frequency corresponds to a frequency of the microwave supplied from the high-frequent power source to the wave guide (usually, the frequency is 2.45 GHz, one of bands defined as ISM bands).
However, the resonant frequency fluctuates in the range of hundreds of megahertz under the influence of physical characteristics of the gas transport tube (relative permittivity, diameter and thickness etc.) and change of relative permittivity of the plasma associated with change of density of the plasma generated within the gas transport tube. In addition, when, for example, a silica tube is used as the gas transport tube, the resonant frequency fluctuates in the range of tens of megahertz under influence of change of relative permittivity of the gas transport tube associated with erosion of the interior surface of the gas transport tube by ions and radicals.
Thus in order to maintain effective generation of plasma, the fluctuation of the resonant frequency needs to be corrected. As a method of correction of the fluctuation of the resonant frequency, for example, it may be proposed to make the frequency of the microwave supplied from the high frequency power source variable. However, according to this method, in order to achieve high output power of the microwave, a large-scale and complex power source is required, which leads to increase of manufacturing cost. Furthermore, it is undesirable to fluctuate the frequency of 2.45 GHz in the range of few hundreds megahertz because of deviance from the statutory ISM bands.
As another method of correction of the fluctuation of the resonant frequency, for example, it may be proposed to make the volume of the microwave cavity variable. However, according to this method, it is necessary to make the side walls of the microwave cavity movable and so on, and thereby the structure of the microwave cavity becomes complex and large-scale, which leads to increase of manufacturing cost. In addition, a means for feedback control of the volume of the microwave cavity is required, which leads to further structural complication and cost up of the microwave plasma generating apparatus. Thus the correction of a resonant frequency in a microwave plasma generating apparatus provided with a conventional cavity resonator was accompanied with problems of complicating the microwave plasma generating apparatus and increasing the manufacturing cost.